Cheaper copies of the world’s biggest-selling drug will roll out across Europe this week after a key European patent for Humira expires Tuesday, but U.S. patients and insurers will have to wait to access less-expensive versions of the blockbuster drug.

The reason: a formidable wall of patents built up by Humira-maker AbbVie Inc., that prevents the developers of “biosimilar” versions launching their products in the U.S.

Biosimilars are near-copies of biologic drugs, such as Humira, that are made from living cells in a process that resembles brewing. They are analogous to generic copies of traditional pill-form medicines.

Biologic drugs are some of the costliest in the world, and the availability of lower-cost versions as patents expire promises big savings. Humira—a drug used to treat diseases from rheumatoid arthritis to gut disorders—alone has more than $18 billion in global sales.

Companies haven’t revealed pricing for the Humira biosimilars, but they’re expected to sell at a 10% to 25% discount to Humira’s $10,000 to $22,000 annual price tag in Europe’s biggest drug markets, according to Alexandra Annis, an analyst at health-care-market intelligence firm GlobalData.

The main U.S. patent for Humira expired in 2016. But AbbVie has obtained more than 100 additional U.S. patents, a number legal experts describe as exceptional for a single drug. The shelf lives of those patents extend into the 2020s and 2030s.

Critics, including lawmakers and industry officials, say AbbVie has created a “patent thicket” that abuses the U.S. patent system in order to preserve its profits.

AbbVie says it is protecting investments it made developing an innovative drug. “There’s nothing about our intellectual property around Humira, or the licensing agreements we’ve done ... that’s anything close to gaming the system,” AbbVie Chief Executive Richard Gonzalez said on a conference call with analysts in July.

“We invested a tremendous amount in research and development” to test Humira in a range of diseases and gain regulatory approval for those uses, Mr. Gonzalez said. “And we’ve improved and refined the manufacturing and the formulation of Humira over time, and there’s nothing inappropriate about protecting that investment in innovation,” he added.

“Europe has a much more robust and effective and time-tested means to challenge the validity of patents,” said Robert Cerwinski, a New York-based intellectual property lawyer specializing in pharmaceuticals at Goodwin Procter LLC. “So the companies were able to challenge AbbVie patents in the so-called patent thicket earlier and more effectively than in the U.S.”

AbbVie said in a statement that it welcomes the introduction of biosimilars in Europe but patients who are stable on their existing drugs shouldn’t be switched to another product for nonmedical reasons.

Amgen Inc. and Novartis AG plan to start selling their biosimilar versions of Humira—Amgevita and Hyrimoz respectively—on Tuesday as soon as the European patent expires. Samsung Bioepis Ltd. and Mylan NV are expected to start selling their versions soon after.

Since Humira generates an estimated $4 billion in annual sales in Europe, its second-biggest market after the U.S., the discounted prices should yield hundreds of millions of dollars in annual savings for the continent’s health systems.

England’s National Health Service, which currently spends more than £400 million ($526 million) a year on Humira, says it hopes to save at least £150 million a year by 2021 by switching to biosimilars.

But in the U.S., extra patents have pushed biosimilar developers to delay bringing their products to market until the next decade.

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Amgen and Boehringer Ingelheim GmbH developed the two Humira biosimilars approved in U.S., but they haven’t so far gone on sale. Amgen settled litigation with AbbVie and agreed to wait until 2023 to launch its version. Boehringer Ingelheim says it plans to introduce a biosimilar Humira in the U.S. before 2023, but it hasn’t specified a date while it defends itself against a patent-infringement lawsuit brought by AbbVie.

At least three more AbbVie rivals-—Samsung Bioepis, Mylan and Novartis AG—have agreed in legal settlements with AbbVie not to sell their coming biosimilars in the U.S. until 2023.

For now, AbbVie has a U.S. monopoly on a drug whose price has risen to more than $60,000 annually for some patients in the 15 years since it launched, and which racked up more than $12 billion in U.S. sales last year.

Slow approval of biosimilars, as well as limited uptake once such drugs do go on sale in the U.S., is costing the American health-care system an estimated $15 billion-$18 billion a year in missed savings, according to Gary Stibel, founder and CEO of the New England Consulting Group.

A decade ago, the best-selling non-biologic drugs in the U.S. had an average of five patents each, according to Lisa Larrimore Ouellette, a law professor at Stanford University. Now, the 12 top-selling drugs in the U.S., many of which are biologics, have an average of 71 patents per drug, according to a recent study from I-MAK, a group that files legal challenges against the validity of brand-name drug patents.

Some elected and appointed officials have raised questions about patents as impediments.

AbbVie in particular has attracted sharp criticism over its patenting activities. In December 2017, Sen. Susan Collins (R-Maine), said AbbVie’s patents have “blocked competitors from coming to the market” and called for ways to counter such practices.

“It’s billions of dollars that this costs the U.S. health-care system,” said Dennis Lanfear, chief executive of Coherus Biosciences, which has pushed back its planned U.S. launch date for a Humira biosimilar to 2022 from 2018 because of AbbVie’s patents. “The market would be served if there was more competition.”